High gradient magnetic separation refers to a procedure for selectively retaining magnetic materials in a chamber or column disposed in a magnetic field. This technique can also be applied to non-magnetic targets labeled with magnetic particles. In one application of this technique a target material, typically a biological material, is labeled by attaching the target material to a magnetic particle. The attachment is generally through association of the target material with a specific binding partner which is conjugated to a coating on the particle which provides a functional group for the conjugation. The material of interest, thus coupled to a magnetic "label", is suspended in a fluid which is then applied to the chamber. In the presence of a magnetic gradient supplied across the chamber, the magnetically labeled target is retained in the chamber; if the chamber contains a matrix, it becomes associated with the matrix. Materials which do not have magnetic labels pass through the chamber. The retained materials can then be eluted by changing the strength of, or by eliminating, the magnetic field. The magnetic field can be supplied either by a permanent magnet or by an electromagnet. The selectivity for a desired target material is supplied by the specific binding-partner conjugated to the magnetic particle. The chamber across which the magnetic field is applied is often provided with a matrix of a material of suitable magnetic susceptibility to induce a high magnetic field gradient locally in the chamber in volumes close to the surface of the matrix. This permits the retention of fairly weakly magnetized particles, and the approach referred to as high gradient magnetic separation (HGMS)
U.S. Pat. No. 4,452,773 ('773) describes the preparation of magnetic iron-dextran microspheres and provides a summary of art describing the various means of preparation of particles suitable for attachment to biological materials. As long ago as 1977, preparation of colloidal iron oxide, gamma-irradiated in the presence of hydrophilic and hydrophobic methacrylate monomers, to provide particles for attachment to biological targets through coupling to immunoglobulin was described (Rembaum, A., et al., Nature (1977) 268:437-438. Various other preparations of magnetic microspheres of various sizes were described by Kronick, P. L., et al, Science (1978) 200:1074-1076 and Widder, K., et al, J Pharm Sci (1979) 68:79-82 and in U.S. Pat. Nos. 3,970,518 and 4,018,886. Particles as large as 100 u have been used. All of these preparations are characterized in the '773 patent as unsatisfactory for general application to HGMS for biological materials for one reason or another.
U.S. Pat. No. 4,230,685 describes an improvement in attaching specific binding agents to the magnetic particles wherein a particle coated with an acrylate polymer or a polysaccharide can be linked through, for example, glutaraldehyde to a preparation of protein A which can then selectively bind antibodies through the Fc portion, leaving the immunoreactive Fab regions exposed. Albumin, rather than polyacrylamide or polysaccharides, is the preferred matrix. A wide size range of particles is disclosed.
In the case of the particles prepared as described in '773, articles of 100-700 angstroms, particularly 300-400 angstroms are intended to be prepared; many of the particles are thus colloidal, and are ferromagnetic with a coating of dextran. The resulting particles are described and claimed as discrete colloidal size particles having a ferromagnetic iron oxide core coated with a polysaccharide derivative having pendant functional groups provided by periodate oxidation. These particles are prepared by mixing an aqueous solution of a ferrous and ferric salt with a solution of the polysaccharide or polysaccharide derivative. After this mixing, alkali is added to cause the formation of the magnetic iron oxide particles to which the polysaccharide or derivative attaches. The resulting particles are separated from excess dextran using gel filtration chromatography. A single peak containing the entire size range of particles is obtained. The polysaccharide is then treated to provide the needed functional groups for conjugation to an immunospecific or other specific binding reagent.
Other polymeric coatings for magnetic particles used in HGMS, or for other biological applications such as NMR imaging, are found in PCT application WO85/04330.
In theory, several types of magnetic particles could be prepared: ferromagnetic particles, superparamagnetic particles and paramagnetic particles. Methods to prepare superparamagnetic particles are described in U.S. Pat. No. 4,770,183. With respect to terminology, as is the general usage in the art:
"Diamagnetic" as used herein, and as a first approximation, refers to materials which do not acquire magnetic properties even in the presence of a magnetic field, i.e., they have no appreciable magnetic susceptibility.
"Paramagnetic" materials have only a weak magnetic susceptibility and when the field is removed quickly lose their weak magnetism. They are characterized by containing unpaired electrons which are not coupled to each other through an organized matrix. Paramagnetic materials can be ions in solution or gases, but can also exist in organized particulate form.
"Ferromagnetic" materials are strongly susceptible to magnetic fields and are capable of retaining magnetic properties when the field is removed. Ferromagnetism occurs only when unpaired electrons in the material are contained in a crystalline lattice thus permitting coupling of the unpaired electrons. Ferromagnetic particles with permanent magnetization have considerable disadvantages for application to biological material separation since suspension of these particles easily aggregate due to their high magnetic attraction for each other.
"Superparamagnetic" materials are highly magnetically susceptible--i.e., they become strongly magnetic when placed in a magnetic field, but, like paramagnetic materials, rapidly lose their magnetism. Superparamagnetism occurs in ferromagnetic materials when the crystal diameter is decreased to less than a critical value. Superparamagnetic particles are preferred in HGMS.
Although the above-mentioned definitions are used for convenience, it will immediately be apparent that there is a continuum of properties between paramagnetic, superparamagnetic, and ferromagnetic, depending on crystal size and particle composition. Thus, these terms are used only for convenience, and "superparamagnetic" is intended to include a range of magnetic properties between the two designated extremes.
The extent of magnetization which is acquired by a particle is a function of its magnetic susceptibility and the applied magnetic field. The magnetization is a function of the resulting magnetic moment and of the volume of the particle. The higher the magnetic moment and the smaller the volume, the higher the magnetization.
Various forms of apparatus for use in HGMS have also been described. Early workers, as exemplified by Molday, R. S., et al, J Immunol Meth (1982) 52:353-367, used simply a tuberculin syringe body across which a magnetic gradient was applied. U.S. Pat. No. 4,738,773 describes a separation apparatus which employs helical hollow tubing made either of stainless steel or Teflon.TM., for example, wherein the helices are placed in an applied magnetic field. Graham, M. D., WO87/01607 and Graham, M. D., et al, U.S. Pat. No. 4,664,796 describe more complex configurations in which the position of the magnetic field can be varied across the separation column. A feature of the Graham et al apparatus (which has been used by others, also) is the inclusion of a matrix which intensifies the localized magnetic gradient as the fluid passes through the interstices of the matrix; this is a necessity for separation of weakly magnetic materials, such as paramagnetic red blood cells. Complex protocols for retention and elution which involve alteration of the position of the magnetic field and alteration of the velocity or viscosity of the carrier fluid are also described. The matrix itself is described as constructed of magnetic wires, fibers, spheres and so forth. Such a description would include, for example, steel wool.
Kronick, U.S. Pat. No. 4,375,407 ('407) describes a device for HGMS where the fluid which contains the particles to be separated, is passed through a filamentary material that has been coated with a hydrogel polymer. According to the disclosure in '407, advantage is taken of the strong magnetic forces produced by the high field gradients at the edges of the filaments which permit particles of even very weak magnetic material to be retained. This advantage of providing a filamentous matrix had been recognized in chemical processing and related methodologies, but, in separations involving biological materials, the filament retains biological entities nonspecifically and furthermore damages them. In part of damage to biological materials in the systems is due to the oxidation (corrosion) of the matrix and the resulting ions in solution, or to the chemical alteration of the magnetic particles to which the biological materials are conjugated. The propensity of the matrices to corrode is intensified in physiological solutions containing saline.
The hydrogel polymer in '407 is for the purpose of overcoming some of these drawbacks. The hydrogel polymer is defined as a polymer which imbibes or absorbs water to the extent of at least 30% of the weight of the polymer. Exemplified are hydrophilic acrylic polymers (advantageously having functional groups for further derivatization). The use of anything other than a hydrophilic hydrogel is indicated to be disadvantageous as resulting in nonspecific adsorption of biological materials. Nevertheless, it is clear that hydrogels cannot protect the filaments of the matrix from corrosion or the passage of the ions formed by this corrosion through the hydrogel into the fluid being passed through the interstices. The function of the hydrogel appears to be associated mainly with elimination of nonspecific binding. Other features of the separation apparatus are standard.
The art thus provides methods for effecting HGMS which are useful, but far from optimal. The present invention is directed to methods and materials which result in more versatile and more effective magnetic separations of biological materials.